How to make enemies stop spawning

So I have an infinite zombie spawning game, much like Call of Duty: Zombies, and I have 8 GameObjects around the map that act as spawners spawning 1 enemy every 11 seconds, or eight enemies every 11 seconds if you count all the spawners, but the problem is, within a minute or so, there are so many enemies that the game lags then crashes, so how can I make enemies stop spawning after so many are spawned then start spawning again when that number is reduced by the player killing them, and how many enemies do you recommend exist at once? Thanks in advance!